The present invention relates to well logging methods and apparatus, and more particularly to methods for analyzing and determining characteristics of gas, oil, and water bearing shaly sand reservoirs, specifically the porosity, clay content, mode of distribution, and hydrocarbon density of such a formation.
In current well/reservoir logging, the density-neutron log cross-plot technique has proven to be the most reliable tool for porosity and clay content determination in oil and water saturated shaly sands. Comparative departures between these logs have proven to be a good shale indicator because the neutron log is more affected by clay than the density log.
In gas bearing reservoirs, however, the density and neutron log responses are greatly influenced not only by porosity and clay content, but also by such variables as gas density, water saturation and invasion of mud filtrate. Gas and light oil have substantially lower hydrogen concentrations. As a result, if gas or light oil is present, the neutron reading will almost certainly be affected. The apparent porosity measured by the neutron tool will then be too low. On the other hand, bulk density as determined by gamma ray density measurements will be too low, causing the apparent porosity as measured by the density tool to be too high. In gas bearing shale formations, the shale effects are opposite to the gas, so the results can be substantially the same as with a clean sand with a higher potential. This can cause the logging results to be overly optimistic. In such conditions, the conventional cross-plot technique for determining clay content can lead to unacceptable errors. An even greater danger of over-estimating clay content can result from using shale indicators such as GR or SP.
In gas bearing shaly sand reservoirs, therefore, the conventional approach of clay content determination from the density-neutron crossplot technique is not applicable. Even if the volume of clay could be precisely determined from some other method (natural gamma ray, SP, etc.), the determination of effective porosity in gas bearing shaly sand would still present a problem since the hydrogen index and the electron density of the fluid in hydrocarbon bearing reservoirs do not produce similar effects on the two logs, and have to be determined before the effective porosity can be computed. Only in the case when the density of hydrocarbons is known and there is practically no invasion, can the clay content and porosity be easily determined through an iterative process which uses density, neutron and resistivity measurements.
As theoretical considerations have thus shown, there is no simple solution to this problem. Rather, a comprehensive solution appears to be required which incorporates several different approaches depending on available geological and logging information.